(a) Field
The present invention relates to plasma display panels (PDPs) and, more particularly, to video brightness correction.
(b) Description of the Related Art
Video displays, such as the subfield type PDP, which divide images per field into a plurality of subfields (“SF”s) with predetermined weights and then display them, are known in the art. These types of video displays, however, tend to have lowered brightness when a display area of the image is increased for each subfield.
FIG. 7 shows a representative subfield sequence when 8 SFs display 256 gray scales, and the numbers correspond to the number of sustain pulses assigned during a sustain period of each SF.
The SF method tends to cause a variation in brightness as a display area on the image is increased. This occurs when the numbers of the sustain pulses, which are assigned according to the display area ratio of the images, are all the same.
FIG. 8 shows such a reduction of the brightness as the display area ratio increases. In more detail, it shows that the brightness is lowered when the area ratio is increased according to the brightness for each SF. This is due to a voltage drop generated by influences of inner impedance of a driving circuit or of wiring impedance and a driving voltage is varied. When the area ratios of the SFs are uniformly decreased, the brightness is also uniformly decreased.
However, problems occur when the area ratios of the respective SFs are substantially different. That is, since the brightness of the SF with a small area ratio is not lowered, but the brightness of the SF with a large area ratio is lowered as shown in FIG. 8, coexistence of the two cases generates different brightness levels for the respective SFs, damaging the original gray scale display characteristics.
A conventional method for sensing the area ratio for each SF, and increasing or decreasing the number of sustain pulses according to the brightness lowered ratio when the area ratio is different for each SF is disclosed by Japanese Publication Application Hei 9-185343. This reference relates to a display device for configuring display fields of one image with a plurality of subfields, weighting an emit period for each subfield, and displaying the gray scales. In this method, a correction period emission is performed by correcting the emit period so that a display load on the whole display area is calculated for each subfield, A predetermined ratio of the brightness of a display cell caused by each subfield may be provided according to the display load of each calculated subfield (refer to FIG. 9). As a result, the brightness ratio between the subfields is constantly maintained to accurately display the gray scales irrespective of differences of the display loads for the respective subfields, thereby effectively maintaining the original grayscale characteristics.
In general, discharges and emissions are generated on the PDP screen by applying sustain pulses to a combination of two electrodes arranged in horizontal rows, such as the case in which the respective combinations of the electrodes are uniformly provided on the screen.
However, when the area ratios are substantially different for the respective SFs with respect to the vertical direction on the screen, the wiring which causes degradation of the brightness is provided in the horizontal row direction. The driving circuit is also configured by dividing the wires in the row direction into a plurality of blocks. Accordingly, places where the brightness is partially degraded and places where no brightness is degraded are mixed, causing a variation in brightness across the display.
FIG. 6 shows an example of a screen that does not vary the load ratios of the SFs because it displays black, which is the lowest gray level, and white, which is the highest gray level.
However, since each row line has a different area ratio for displaying white, the brightness of a line with a large area ratio is lowered, and the brightness of a screen with a small area ratio is not lowered. A brightness difference, therefore, is generated as shown in FIG. 6 for the black and white grayscales.